FIG. 1 shows the general logical architecture proposed for UMTS/GPRS and also suitable for other systems. A mobile station (MS) 1 can communicate by radio with one or more base stations (BS) 2. The respective network element in GPRS is called a BTS (base transceiver station). Each base station is linked to a single radio network controller (RNC) 4. The respective network element in GPRS is called a BSC (base station controller). Each RNC can be linked to one or more BSs. Each RNC is linked to a core network (CN) 5. The CN includes one or more serving nodes that can provide communication services to a connected mobile station, for example a mobile switching centre (MSC) 7 and a serving GPRS (general packet radio service) support node (SGSN) 8. These units are connected to the RNCs. The CN 5 is also connected to other telecommunications networks such as a fixed line network 9, other mobile networks (e.g. another core network 12, not shown in detail in FIG. 1) or packet data networks 10, 11 such as the internet or proprietary networks to allow onward connection of communications outside the UMTS network. The CN also includes other units such as a home location register (HLR) 13 and a visitor location register (VLR) 14 which help to control access to the network. The HLR 13 stores the subscription details of mobile station subscribers to that CN itself. The VLR stores information on mobile stations that are currently attached to the CN. The BSs and the RNCs constitute a UMTS terrestrial radio access network (UTRAN). Each core network includes one or more charging gateway functionality entities 15, 16 and a billing system 17, 18 for performing billing operations. (In GPRS, for example, If the charging gateway functionality is embodied in a separate physical unit then the corresponding network element is known as a charging gateway (CG)). Each network entity generating charging information (like GGSN and SGSN in GPRS) is linked at least to one charging gateway functionality (CGF), but for redundancy reasons typically to several CGFs. (The charging gateways may also be linked together.) When a mobile station is operating in another core network from the one to which it is subscribed (its home network), that other core network can communicate charging information to the home network by means of the charging gateways and billing systems so that the home network can bill the subscriber for his use of the other core network.
In the core network each serving node such as an MSC or SGSN can provide a set of services to the mobile station. For example:                An MSC can provide circuit switched (CS) communications, for example for speech, fax or non-transparent data services, and therefore has a link to other entities in the circuit switched domain such as other CS mobile networks such as GSM (Global System for Mobile communications) and CS fixed wire networks such as conventional voice telephony networks.        An SGSN can provide packet switched (PS) communications, for example for packet data protocol (PDP) contexts for internet protocol (IP) data transmission, and therefore has a link to other entities in the packet switched domain such as GPRS-equipped GSM networks and the internet. The packet switched services may include traditional data services such as file transfer, e-mail and world-wide web (WWW) browsing and derived data services such as voice-over-IP (e.g. by means of the H.323 protocol).        
The division of services between serving nodes is specified in the system specification and is tied to the assumed network architecture. There may be other nodes than the MSC or SGSN providing overlapping or additional services.
When a mobile station begins operating for communication in the coverage area of the core network it first undergoes a process of attachment to the core network. In that process the mobile station indicates its identity, and then undergoes a process of attachment to the network. The core network obtains subscription information for the mobile station from the HLR of the core network to which the mobile station is subscribed. The subscription information includes access information indicating the services that the mobile station is entitled to receive (e.g. the access points—for example in a GGSN—to which the mobile station may have access), and billing information indicating the method by which the subscriber is billed (e.g. normal subscription, pre-paid subscription, hot billing subscription or flat rate subscription; and whether billing is to be dependant on the access point that is used). Using that information the core network can provide services to the mobile station and cause the subscriber to be billed accordingly. After having attached to the network the mobile station may communicate to the core network its need for communication services, for example involving activation of a PDP context in the GPRS system.
In some systems, e.g. GPRS, CDR (call detailed record) tickets or other charging messages are collected by the charging gateway functionality and sent onwards as a file towards the appropriate billing system periodically, and after some delay. For example, such files may be sent every 10 or 30 minutes. In hot billing the messages are typically sent promptly towards the appropriate billing system or billing server after no delay or only a few seconds' delay. This can enable services such as advice of charge (AoC) and pre-paid subscriptions to be provided more effectively.
The core network includes gateway equipment 19, 20, 21 for interfacing with the other networks 9, 10, 11. Where the respective other network is a packet switched network (e.g. networks 10, 11) the gateway equipment is a GGSN (gateway GPRS support node), which interfaces between the SGSN 8 and the respective network. During a communication session and/or after a session has been completed the GGSN through which the session was routed generates one or more CDR ticket messages which are directed to the appropriate charging system so that the subscriber can be billed for the session.
Under a so-called normal charging arrangement a subscriber is charged simply on the basis of the duration of sessions initiated by his mobile station, or a like measure such as the amount of data sent and/or received by the mobile station (e.g. the number of data octets sent and received). This arrangement is served efficiently by the above charging method. However, alternative charging arrangements are becoming increasingly popular with subscribers. These arrangements include:    1. pre-paid billing, in which a subscriber makes a prepayment for sessions in advance and the costs of sessions that are made are deducted from the prepayment; and    2. flat rate subscription, in which a subscriber is charged a fixed amount irrespective of how many sessions he makes.
The inventors of the present invention have identified that the charging of the latter arrangements could potentially be achieved more efficiently if there were a means whereby the sending of CDR ticket messages (or other analogous charging messages) could be better adapted to those arrangements.